Capillary waves and air-sea gas transfer

Abstract

The effects of capillary waves are considered on the transfer of gas into (or out of) solution through a gas-liquid interface. The bulk liquid is assumed to be otherwise motionless in the analysis of a preliminary problem; in this problem, a concentration boundary layer is developed as a consequence of a first-order chemical reaction that is assumed to deplete the dissolved gas in the liquid. The reaction rate determines the asymptotic thickness of the concentration boundary layer. It is shown that gas transfer through the concentration boundary layer is most enhanced by the presence of capillary waves when there is vigorous removal of dissolved gases by chemical reaction - i.e. when the reaction is fast and the boundary layer is thin. The results of this theory are then measured against gas transfer through a turbulent, sheared interface in the context of a surface renewal model. Here it is the exchange, from time to time, of fluid between the interface and the bulk that leads to the development of a thin concentration boundary layer when the bulk fluid is not saturated with dissolved gas. Capillary waves are shown to thicken the concentration boundary layer at the interface and to increase the rate of gas transfer.